Cross-linking of bulk fluoropolymers, including thermoplasts and elastomers, is one of the most common techniques in polymer science to stabilize shape, improve mechanical properties and fix structure of shaped articles and accordingly many methods have been reported on how to achieve well-defined cross-linking processes.
Within this scenario, sulfonyl azide groups have attracted much attention for being either incorporated as cure-site in polymeric chain or used as coupling agents, because of their peculiar reactivity. Actually, it is well known that sulfonyl azide group decomposes thermally or under UV radiation to form a nitrene intermediate, which is capable of extracting a hydrogen atom or inserting into a saturated carbon-hydrogen bond or coupling with another nitrene moiety to form a diazo compound.
Chemistry of azide groups, and more particularly of sulfonazide groups, has thus found application in the domain of fluoropolymers for effecting crosslinking of the same. Thus, WO 2010/021962 (3M INNOVATIVE PROPERTIES CO) 25 Feb. 2010 discloses fluoropolymers comprising azide groups (different from sulfonazide moieties) which are generally present as end-groups and which can be introduced in the fluoropolymer as a result of the use of an azide compound as radical initiator, using an azide group-containing chain transfer agent or by nucleophilic deplacement of leaving groups present in the polymer by use of azides.
Also, US 2007166838 (KONINKL PHILIPS ELECTRONICS NV) 19 Jul. 2007 discloses, inter alia, crosslinkable polymer compositions comprising a VDF-TrFE polymer and, among others crosslinking agents, an azide compound, e.g. 4,4-dithiobisphenyl-azide, 3,3′-diazododiphenyl sulphone.
Further, in addition, sulfonyl azide-containing molecules are otherwise recognized as quite robust compounds, e.g. stable in usual polymerization conditions, including in aqueous media, so that monomers carrying such moieties have already been used in standard free radical polymerization processes.
Sulfonyl azide-containing monomers have been already incorporated in fluoropolymers. Thus, U.S. Pat. No. 6,365,693 (DUPONT DOW ELASTOMERS LLC) 2 Apr. 2002 discloses fluoroalkanesulfonylazide unsaturated compounds which can be used as functional monomers in fluoropolymers, in particular in fluoroelastomers, including VDF-based fluoroelastomers. These compounds comply with formula CF2═CF—(O)p—Rf—(CH2)n—S(O)qN3, wherein p=0 or 1; n=0-4; q=1 or 2; and Rf is a C1-C16 perfluoroalkyl of perfluoroalkoxy group. Embodiments with p=1, i.e. perfluorovinylethers derivatives are specifically disclosed: CF2═CF—O—CF2—CF(CF3)—O—CF2CF2—SO2N3, CF2═CF—O—CF2CF2—SO2N3, CF2═CF—O—CF2CF2CF2—SO2N3, CF2═CF—O—CF2CF2CF2CF2—SO2N3. Other embodiments disclosed are those wherein p=0 and an oxygen atom is comprised in the Rf moiety, including notably compound: CF2═CF—CF2CF2—O—CF2CF2—SO2N3.
Similarly, WO 2010/147697 (DUPONT PERFORMANCE ELASTOMERS) 23 Dec. 2010 discloses certain curable compositions based on fluoroelastomer containing azide groups, including through the incorporation of recurring units comprising azide groups; as per the description of azide-containing monomers, this document refers back to document U.S. Pat. No. 6,365,693 (DUPONT DOW ELASTOMERS LLC) 2 Apr. 2002 and mentions perfluorinated vinyl ether compounds, including notably CF2═CF—O—CF2—CF(CF3)—O—CF2CF2—SO2N3, CF2═CF—O—CF2CF2—SO2N3, CF2═CF—O—CF2CF2CF2—SO2N3, CF2═CF—O—CF2CF2CF2CF2—SO2N3.
Nevertheless, the chemistry and incorporation of these monomers wherein the ethylenically unsaturated moiety is of formula CF2═CF—O— is strongly influenced by this vinyl ether character, so that effectiveness in incorporation in the fluoropolymer chain by radical addition over transfer and beta-scission phenomena, and chain transfer phenomena, monomer distribution and molecular weight in copolymers comprising the same and cross-linking ability thereof might be negatively affected.